Remove bogus asserts

...regression introduced with b0ef5cf2 "sal: add
some argument checking assertions for strings and buffers" not being aware of
18699039 "Clean up Mac _imp_getProcessLocale:
Introduces OUStringBuffer::appendUninitialized."

Change-Id: I828d98eb52f57f4e39e71ded39ef034e1788f4d1

sal/rtl/strbuf.cxx

@@ -105,7 +105,6 @@ void SAL_CALL rtl_stringbuffer_insert( rtl_String ** This,
     assert(This);
     assert(capacity && *capacity >= 0);
     assert(offset >= 0 && offset <= (**This).length);
-    assert(len == 0 || str != nullptr);
     assert(len >= 0);
     sal_Int32 nOldLen;
     sal_Char * pBuf;

sal/rtl/ustrbuf.cxx

@@ -129,7 +129,6 @@ void SAL_CALL rtl_uStringbuffer_insert( rtl_uString ** This,
     assert(This);
     assert(capacity && *capacity >= 0);
     assert(offset >= 0 && offset <= (**This).length);
-    assert(len == 0 || str != nullptr);
     assert(len >= 0);
     sal_Int32 nOldLen;
     sal_Unicode * pBuf;